Method for producing catalyst for fuel cell

ABSTRACT

The present invention provides a method for producing a catalyst for a fuel cell comprising:
     a step for forming an inverted micelle consisting of an aqueous solution containing the iridium compound clathrated by a surfactant, by mixing an organic solvent containing said surfactant, and the aqueous solution containing said iridium compound;   a step for forming a fine iridium particle aggregate by insolubilization treatment of said iridium compound;   a step for impregnating said fine iridium particle aggregate with an aqueous solution containing a platinum compound;   a step for obtaining a solution containing the inverted micelle clathrating the fine iridium particle aggregate containing platinum by reducing said platinum compound and depositing platinum metal in said fine iridium particle aggregate;   a step for supporting said fine iridium particle aggregate containing platinum on a conductive carrier by dispersing said conductive carrier in said solution; and   a step for firing the conductive carrier whereon said fine iridium particle aggregate containing platinum is supported.

TECHNICAL FIELD

The present invention relates to a method for producing a catalyst for afuel cell, in particular relates to a catalyst for a fuel cell highlydispersed and supported with an alloy containing platinum and iridium.

DESCRIPTION OF RELATED ART

A fuel cell is a clean power generation system with little adverseeffect on global environment, whose product by an electrode reaction isprincipally water. As a fuel cell, various types of fuel cells have beenproposed including a proton-exchange membrane fuel cell, a solid oxidetype fuel cell, a molten carbonate type fuel cell, a phosphoric acidtype fuel cell, and the like.

Among the fuel cell, a proton-exchange membrane fuel cell (PEFC) hasbeen expected as a power source for a movable body such as anautomobile, and development thereof have been progressing because ofbeing operable at relatively low temperature. A proton-exchange membranefuel cell usually has laminated composition of a catalyst layer, a gasdiffusion layer and a separator in this order at the both sides of asolid polyelectrolyte membrane. The separator is arranged with a powercollection body for taking out electrons generated.

The separator is formed with a gas flow channel at the surfacecontacting with the gas diffusion layer. To the gas flow channel at anoxygen electrode side, an oxidizing agent such as air or oxygen gas issupplied, and to the gas flow channel at a fuel electrode side, a fuelgas such as hydrogen gas is supplied. Reaction gas supplied to the gasflow channel reach the catalyst layer through the porous gas diffusionlayer to generate electrons by the following electrode reaction. Theelectrons generated move to the power collection body through the gasdiffusion layer and the separator, and are taken out to an externalcircuit.Cathode reaction (Oxygen electrode): 2H⁺+2e⁻+1/2O₂→H₂OAnode reaction (Fuel electrode): H₂→2H⁺+2e⁻  [Chemical Equation 1]

The catalyst layer contains a catalyst for promoting the electrodereaction. As the catalyst for promoting the electrode reaction in a fuelcell, one composed of platinum supported on a carrier has been studied.However, use of platinum metal as a catalyst component has a problem ofplatinum elusion in power application, resulting in lowering of cellperformance. As technique to improve stability of the catalyst, there istechnique using an alloy composed of platinum and iridium supported on acarrier, as a catalyst (JP-A-2004-22503, paragraph 0018)

DISCLOSURE OF THE INVENTION

In a conventional method for production, platinum raw material andiridium raw material are separately supplied to a carrier, and bysubsequent firing, platinum and iridium are commingled to form aplatinum-iridium alloy. For example, platinum is supported on a carrierby using a solution containing platinum, and then by using a solutioncontaining iridium, iridium is supported on the carrier. However, when aplatinum-iridium alloy is prepared by such technique, because platinumand iridium are separately present on a conductive carrier, progress ofalloy making of platinum and iridium by firing is difficult, and firingat high temperature is required. Consequently, high temperature firingresults in not only coarsening of a platinum-iridium alloy and loweringcatalytic performance but also increasing difficulty in obtaining aplatinum-iridium alloy with homogeneous composition.

In view of the above problems, it is an object of the present inventionto provide a method for producing a catalyst for a fuel cell composed ofa platinum-iridium alloy with improved catalytic activity.

The present invention relates to a method for producing a catalyst for afuel cell comprising:

-   1. A method for producing a catalyst for a fuel cell comprising: a    step for forming an inverted micelle consisting of an aqueous    solution containing the iridium compound clathrated by a surfactant,    by mixing an organic solvent containing said surfactant, and the    aqueous solution containing said iridium compound;-   a step for forming a fine iridium particle aggregate by    insolubilization treatment of said iridium compound;-   a step for impregnating said fine iridium particle aggregate with an    aqueous solution containing a platinum compound;-   a step for obtaining a solution containing the inverted micelle    clathrating the fine iridium particle aggregate containing platinum    by reducing said platinum compound and depositing platinum metal in    said fine iridium particle aggregate;-   a step for supporting said fine iridium particle aggregate    containing platinum on a conductive carrier by dispersing said    conductive carrier in said solution; and-   a step for firing the conductive carrier whereon said fine iridium    particle aggregate containing platinum is supported.

Further other objects, features and characteristics of the presentinvention will be clear by referring to explanation below and preferableEmbodiments exemplified in the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process chart showing one Embodiment of the presentinvention.

FIG. 2 is a conceptual diagram of an inverted micelle.

FIG. 3 shows X-ray diffraction spectra of a catalyst for a fuel cellprepared in Example 1 and a catalyst for a fuel cell prepared inComparative Example 1.

BEST EMBODIMENT FOR CARRYING OUT THE INVENTION

The outline of a method for production of the present invention isbriefly explained using FIG. 1. First, by mixing an organic solventcontaining a surfactant and an aqueous solution containing an iridiumcompound, an inverted micelle consisting of an aqueous solutioncontaining the iridium compound clathrated by a surfactant is formed.Then an aggregate of fine iridium particles composed of such asIr(OH)₃(H₂O)₃, is formed in the inverted micelle, by insolbilizationtreatment of the iridium compound present inside the inverted micelle,by the addition of a precipitating agent, and the like. Subsequently, anaqueous solution containing a platinum compound is impregnated into thefine iridium particle aggregate, by supplying an aqueous solutioncontaining a platinum compound inside the inverted micelle. After that,by reducing the platinum compound, which is impregnated inside the fineiridium particle aggregate, by the addition of a reducing agent, and thelike, platinum metal is deposited, whereby the fine iridium particleaggregate containing platinum is obtained. Because the resultant fineiridium particle aggregate containing platinum is present inside theinverted micelle, the particles can homogeneously be dispersed bysuppressing coagulation of the particles in a solution. Then, the fineiridium particle aggregate containing the platinum can be supported on aconductive carrier in highly dispersed state, by dispersing theconductive carrier in the solution containing the fine iridium particleaggregate including platinum.

As described above, in the present invention, by preparing the fineiridium particle aggregate containing platinum in advance, andsupporting on the conductive carrier, alloy making becomes easiercompared with the case in making platinum and iridium separatelysupported, and significant reduction of sintering of theplatinum-iridium alloy, by thermal energy in firing, is possible. Inaddition, advantage of producing particles using an inverted micellemethod includes easy control of particle size. Use of the invertedmicelle method is capable of controlling particle size in nanometerorder.

Furthermore, in the present invention, platinum is deposited afterdepositing iridium first inside the inverted micelle. In certain caseswhen platinum is deposited before iridium, or platinum and iridium aredeposited at the same time, platinum tends to be precipitated more andiridium is little precipitated. Therefore, deposition of platinum andiridium inside the inverted micelle with homogeneous composition wasdifficult, and ratio of platinum and iridium supported on a carrier wasnot stable and composition could not be made homogeneous. Noticing onthese points, in the present invention, it has been found that bydepositing iridium first, alloy particles having a composition withhomogeneous ratio of platinum and iridium can be supported on aconductive carrier.

A method of the present invention is explained in detail in the order ofevents.

First, by mixing an organic solvent containing a surfactant, and anaqueous solution containing an iridium compound, an inverted micelleconsisting of an aqueous solution containing the iridium compoundclathrated by a surfactant is formed. FIG. 2 shows a conceptual diagramof the inverted micelle. By mixing the aqueous solution containing theiridium compound and the organic solvent containing the surfactant, theinverted micelle consisting of an aqueous solution 30 containing theiridium compound clathrated by the surfactant 20 is formed in theorganic solvent. In this connection, a conceptual diagram of theinverted micelle shown in FIG. 2 is simplified for convenience ofexplanation, however, technical scope of the present invention is by nomeans limited to the Embodiment shown here.

The surfactant used for forming the inverted micelle is not especiallylimited. For example, surfactants such as polyethylene glycolmono(4-nonyl)phenylether, polyoxyethylene nonylphenylether, magnesiumlaurate, zinc caprate, zinc myristate, sodium phenylstearate, aluminiumdicaprylate, tetraisoamylammonium thiocyanate, n-octadecyltri(n-butyl)ammonium formate, n-amyl tri(n-butyl)ammonium iodide, sodiumbis(2-ethylhexyl)succinate, sodium dinonylnaphthalene sulfonate, calciumcetyl sulfate, dodecylamine oleate, dodecylamine propionate, cetyltrimethylammonium bromide, stearyl trimethylammonium bromide, cetyltrimethylammonium chloride, stearyl trimethylammonium chloride, dodecyltrimethylammonium bromide, octadecyl trimethylammonium bromide, dodecyltrimethylammonium chloride, octadecyl trimethylammonium chloride,didodecyl dimethylammonium bromide, di(tetradecyl)dimethylammoniumbromide, didodecyl dimethylammonium chloride,di(teradecyl)dimethylammonium chloride, and(2-octyloxy-1-octyloxymethyl)polyoxyethylene ethyl ether can be used.

The organic solvent is also not especially limited. For example, organicsolvents such as cyclohexane, methylcyclohexane, cycloheptane, heptanol,octanol, dodecyl alcohol, cetyl alcohol, isooctane, n-heptane, n-hexane,n-decane, benzene, tolueneandxylene can be used.

The organic solution and the surfactant preferably are stirred beforesubjecting to mixing with the aqueous solution containing the iridiumcompound. Use amount of the surfactant and the organic solvent is notespecially limited, however, concentration of the surfactant preferablyis 0.01 to 1 mol/l, and more preferably is 0.1 to 0.3 mol/l.

The iridium compound is raw material of iridium composing theplatinum-iridium alloy particles. The iridium compound preferably has acertain degree of water solubility. Kind of the iridium compound is notespecially limited, as long as suppliable inside the inverted micelle asan aqueous solution. The iridium compound preferably is an iridiumcomplex, including specifically [Ir(OH)₆]³⁻, namely iridium coordinatedwith 6 hydroxide ions.

A method for preparing the aqueous solution containing the iridiumcompound is not especially limited, and may be selected, as appropriate,in response to the iridium compound to be used. As an example of amethod for preparing the aqueous solution containing the iridiumcompound, iridium chloride is mixed with a basic compound containing ahydroxyl group of 5 to 10 times moles of iridium atoms contained in theiridium chloride, and then it is causing a reaction at a temperature of30 to 50° C. for 1 to 5 hours.

As iridium chloride, a compound formable a complex such as [Ir(OH)₆]³⁻,by reacting with OH⁻ in an aqueous solution be preferable. For example,such as hexachloroiridium acid (H₂[IrCl₆], H₃[IrCl₆]), potassiumhexachloroiridate, (K₂[IrCl₆], K₃[IrCl₆]), sodium hexachloroiridate (Na₂[IrCl₆]), diammonium hexachloroiridate ((NH₄)₂[IrCl₆]), and triammoniumhexachloroiridate ((NH₄)₃[IrCl₆]) are included.

The basic compound having a hydroxyl group is a compound having ahydroxyl group in the compound, wherein this hydroxyl group acts as abase by being freely present in water. For example, alkali metalhydroxide such as sodium hydroxide, and potassium hydroxide; alkaliearth metal hydroxide such as calcium hydroxide and magnesium hydroxideare included. Tetramethylammonium hydroxide may also be used, too. Inview of decreasing incorporation amount of metallic impurity such assodium or potassium, tetramethylammonium hydroxide is preferable.

Concentration of the iridium compound in the aqueous solution containingthe iridium compound preferably is 0.1% by mass to 30% by mass, and morepreferably is 0.5% by mass to 3% by mass. Concentration of the iridiumcompound less than 0.1% by mass relatively reduces the amount of iridiumwhich can be added in the solution of the inverted micelle. To maintaina small micelle diameter, water content to be poured preferably is keptequal to or smaller than a certain amount, and increased amount of watercontent to be poured tends to increase particle diameter of theplatinum-iridium alloy finally obtained. Concentration of the iridiumcompound over 30% by mass reduces water content, which makes a metalsalt deposited and could make formation of the inverted micelledifficult. In addition, it tends to increase particle diameter of finemetal particles. However, technical scope of the present invention is byno means limited to these ranges, and in some cases, concentrationoutside the range may be allowed.

Then, after formation of the inverted micelle as described above, thefine iridium particle aggregate is formed by insolubilization treatmentof the iridium compound in the inverted micelle. The fine iridiumparticle aggregate formed by insolubilization treatment of the iridiumcompound is in the inverted micelle formed by a surfactant. In addition,Other than the fine iridium particle aggregate, an aqueous solution iscontained in the inverted micelle formed in an organic solvent.

As an Embodiment of insolubilization treatment of the iridium compound,a method for forming the fine iridium particle aggregate by adding aprecipitating agent to the aqueous solution containing the iridiumcompound is included. The precipitating agent used here preferably isselected depending on kind of the iridium compound. As a typical exampleof the precipitating agent, one or more kinds of acids selected from agroup comprising hydrochloric acid, nitric acid, sulfuric acid, andacetic acid are included.

When the acid is added, it preferably is added so that pH in the aqueoussolution containing the iridium compound is about 7 to 8.5. When pH isover 8.5, fine iridium particles could not sufficiently be deposited.When the acid is added as much as pH becomes lower than 7, the producingfine iridium particles could be reduced.

Composition of the producing fine iridium particles in the invertedmicelle is not especially limited, however, it preferably is a hydrateof a hydroxide such as Ir(OH)₃(H₂O)₃. When a formed fine particle is ahydrate of iridium hydroxide, the fine iridium particle aggregate withsmall particle diameter is formed, which results in sufficient contentof the aqueous solution inside the fine iridium particle aggregate whenthe aqueous solution containing the platinum compound is supplied at alater step, and deposition of platinum in highly dispersed state insidethe fine iridium particle aggregate is possible.

Then, the aqueous solution containing the platinum compound isimpregnated in the fine iridium particle aggregate. To impregnate theaqueous solution containing the platinum compound into the fine iridiumparticle aggregate present inside the inverted micelle, the aqueoussolution containing the platinum compound is supplied into the aqueoussolution containing the inverted micelle, followed by stirring to makethe aqueous solution penetrated inside the inverted micelle. The aqueoussolution containing the platinum compound penetrated in the invertedmicelle is impregnated inside the fine iridium particle aggregate. Inthis time, when the iridium fine particle is a hydrate of a hydroxidesuch as Ir(OH)₃(H₂O)₃, the aqueous solution containing the platinumcompound is easy to be penetrated inside the fine iridium particleaggregate, like water is absorbed in a sponge substance.

The platinum compound is raw material of platinum composing theplatinum-iridium alloy particles. The platinum compound preferably has acertain degree of water solubility. Kind of the platinum compound is notespecially limited, as long as it is suppliable inside the invertedmicelle as an aqueous solution. A typical example of the platinumcompound includes dinitrodiamine platinum Pt(NO₂)₂(NH₃)₂ or H₂PtCl₆, andthe like. Because crystal of dinitrodiamine platinum is fundamentallyinsoluble in water, when dinitrodiamine platinum is used, it preferablyis converted to water soluble by dissolving in nitric acid and added asan aqueous solution of dinitrodiamineplatinum nitrate.

Concentration of platinum in the aqueous solution containing theplatinum compound preferably is 0.01% by mass to 10% by mass, and morepreferably is 0.5% by mass to 3% by mass. However, technical scope ofthe present invention is by no means limited to these ranges, and insome cases, concentration outside the range may be allowed.

After impregnation of the aqueous solution containing the platinumcompound into the fine iridium particle aggregate, platinum metal isdeposited in the fine iridium particle aggregate by reducing theplatinum compound, to yield the fine iridium particle aggregatecontaining platinum. By this step, platinum can be deposited inside thefine iridium particle aggregate in highly dispersed state.

A method for depositing platinum metal inside the fine iridium particleaggregate may be selected, as appropriate, depending on kind of theplatinum compound. For example, platinum metal is deposited in the fineiridium particle aggregate by reducing the platinum compound using areducing agent. The reducing agent used here includes one or more kindsselected from a group comprising N₂H₄, NaBH₄ and H₂ gas. In selectingthe reducing agent, easiness of reducing treatment or kind of the fineiridium particles preferably is considered. For example, deposition ofplatinum metal by N₂H₄ is relatively difficult, and careful control ofreductive reaction rate is preferable. Too rapid reductive reaction ratetends to easily coagulate platinum particles and could generatelocalization of platinum atoms. In consideration of easiness of control,NaBH₄ or H₂ is preferable. In addition, when H₂IrCl₆ is used as iridiumraw material, reduction by NaBH₄ or H₂ gas is effective.

Aiming at improvement of catalytic activity, durability and stabilityagainst CO etc. of the resultant catalyst for a fuel cell, transitionmetal may further be commingled in the fine iridium particle aggregatecontaining platinum. In such a case, an aqueous solution containingtransition metal may further be impregnated inside the fine iridiumparticle aggregate containing platinum, to deposit the transition metalinside the fine iridium particle aggregate containing platinum. Byimpregnation of the aqueous solution containing desired transitionmetal, similarly as in the case of impregnation of the aqueous solutioncontaining platinum, multi-component alloy particles composed ofplatinum, iridium and desired transition metal can be produced. In thecase of producing the multi-component alloy particles, whether any ofplatinum and transition metal is impregnated in advance inside the fineiridium particle aggregate is not especially limited. When allowable,preparation of alloy particles may be tried using an aqueous solutioncontaining both platinum and desired transition metal. Preferably, afterplatinum is first impregnated, desired transition metal is impregnated.Namely, a method of the present invention desirably further has a stepfor impregnating the fine iridium particle aggregate containing platinumwith an aqueous solution containing transition metal, and a step fordepositing the transition metal inside the fine iridium particleaggregate containing platinum. By these steps, transition metal can becontained in the fine iridium particle aggregate containing platinum,and alloy particles having composition of homogeneous ratio of platinum,iridium and transition metal can be obtained, in the resultant catalystfor a fuel cell.

Transition metal preferably is subjected to deposition treatment,similarly as in impregnation of platinum, by dissolving a water solublecompound containing transition metal as raw material, and supplyingthereof into particles containing platinum and iridium. A method fordepositing transition metal other than platinum may be selecteddepending on characteristics of a compound containing the transitionmetal. When the compound to be deposited by reduction is used as rawmaterial, reduction treatment may be carried out by the addition of areducing agent, and the like, and when the compound to be deposited byspecified precipitating agent is used, the precipitating agent maybeadded. When an effective method is present to deposit both of a compoundcontaining platinum and a compound containing transition metal, bothplatinum and transition metal may be deposited by deposition treatmentat one time.

Kind of transition metal is not especially limited, however, inconsideration of catalytic performance of the producing platinum-iridiumalloy particles, transition metal preferably is one or more kindsselected from a group comprising chromium, manganese, iron, cobalt,nickel, rhodium and palladium. The compound containing the transitionmetal is not especially limited, and includes inorganic salts such asnitrate, sulfate, ammonium salt, amine, carbonate, bicarbonate, haloidsalt, nitrite, and oxalate containing a transition metal element;carboxylate such as formate and hydroxide, alkoxide, oxide, and the likecan be exemplified. A reducing agent and a precipitating agent for thecompound containing the transition metal may also be selected dependingon kind of the compound, and similar ones as described above may also beused.

The fine iridium particle aggregate containing platinum obtained asabove is clathrated by the inverted micelle formed in an organicsolvent. According to a method of the present invention, then by mixingthe solution containing the inverted micelle clathrating with the fineiridium particle aggregate containing platinum with the conductivecarrier, the fine iridium particle aggregate containing platinum issupported on the conductive carrier.

The conductive carrier is not especially limited, however, those mainlycomposed of carbon preferably is used. Specifically, Ketjenblack, BlackPearls, graphitized carbon and graphitized Black Pearls, and thoseobtained by subjecting these to graphitization treatment at hightemperature are included. Fouling at the carrier surface may be washedusing an alkali solution of such as sodium hydroxide, potassiumhydroxide, and calcium hydroxide. In addition, BET specific surface areaof the conductive carrier preferably is not lower than 50 m²/g, and morepreferably is 250 to 1600 m²/g.

The conductive carrier preferably is mixed in dispersed state in anorganic solvent. As the organic solvent, a similar organic solvent asused in forming the inverted micelle is used. To enhance dispersionproperty between the conductive carrier and the platinum-iridium alloyparticles, the same organic solvent preferably is used. On theconductive carrier, explanation has already been given in the first ofthe present invention, therefore, explanation here is omitted. Fordispersion of the conductive carrier into the organic solvent, a stirrermay be used, and a method for ultrasonic dispersion and the like mayalso be used.

By mixing the organic solvent containing the conductive carrier, and thesolution containing the inverted micelle clathrating with the fineiridium particle aggregate containing platinum, the fine iridiumparticle aggregate containing platinum is supported on the conductivecarrier.

As a method for the mixing, mixing the conductive carrier into thesolution of the inverted micelle using well-known stirring equipmentsuch as ultrasonic and a homogenizer, subsequently mixing and stirringthe solution and causing to a reaction at 70 to 100° C. for 3 to 12hours to support the aggregate on the conductive carrier be preferable.According to the condition, supporting of the aggregate on the carriersurface can be secured.

In addition, collapsing treatment of the inverted micelle desirably iscarried out after the mixing, and by this procedure, supporting of thefine iridium particle aggregate containing platinum on the conductivecarrier can be promoted. Collapsing treatment of the inverted micellemay be selected in response to kind of a surfactant used. For example,by the addition of an alcohol such as methanol into the solution of theinverted micelle, mixed with the conductive carrier, the invertedmicelle is collapsed.

In the present invention, by controlling supplying amount of the aqueoussolution or the surfactant, size of the inverted micelle can becontrolled, and particle diameter of the resultant fine iridium particleaggregate containing platinum, and in turn platinum-iridium alloyparticles can relatively easily be controlled. Composition of platinumand iridium in the fine iridium particle aggregate containing platinumis not especially limited. Increased size of the inverted micelle bythis aqueous solution, when the aqueous solution containing the platinumcompound is added, it is possible to generate a moiety wherein aplatinum component is relatively rich at the surface of theplatinum-iridium fine particle aggregate. Even in producing theplatinum-iridium alloy particles by firing the platinum-iridium fineparticle aggregate wherein the component is inhomogeneous like this,coarsening of particles by firing can be suppressed compared with thecase in producing by a conventional method, because far more homogeneousprogress of platinum and iridium has been obtained.

After supporting the fine iridium particle aggregate containing platinumon the conductive carrier, solid content is separated by filtration andthe resultant solid content is dried. A method for separation or dryingis not especially limited. For example, drying is carried out underreduced pressure by raising ambient temperature surrounding solidcontent. In some cases, drying may be carried out during a firing step,without carrying out a drying step.

Subsequently, solid content is fired, and by making an alloy of the fineiridium particle aggregate containing platinum, a catalyst for a fuelcell, composed of the platinum-iridium alloy particles supported on theconductive carrier is obtained.

Firing condition is not especially limited. For example, firing iscarried out at 200 to 950° C. for 1 to 4 hours. In addition, firingpreferably is carried out under inert gas atmosphere such as argon andhelium.

When an iridium component to be dried and fired is a hydrate of ahydroxide such as Ir(OH)₃(H₂O)₃, particles significantly shrink by heattreatment and thus particle diameter becomes far smaller, bringing aboutmerit of easy immobilization of platinum and other transition metalinside iridium.

By the method described above, a catalyst for a fuel cell composed ofthe compositionally-homogeneous platinum-iridium alloy particles highlydispersed and supported on the conductive carrier is obtained.

In this connection, any of the following is allowed in theplatinum-iridium alloy particles: eutectic alloy, kind of a mixture ofseparate crystals of component elements; a solid solution made bycomplete melting of component elements each other; those whereincomponent elements form an intermetallic compound or a compound betweenmetal and non-metal, and the like. However, in consideration ofcatalytic activity, durability, and the like of the resultant catalystfor a fuel cell, a solid solution made by complete melting of componentelements each other is desirable in the platinum-iridium alloyparticles.

In addition, the platinum-iridium alloy particles supported on theconductive carrier preferably have homogeneous composition. When thereis no moiety wherein a platinum component is excessively present, or aniridium component is excessively present, and platinum and iridium arehomogeneously dispersed, merit of using an alloy as a catalyst can fullybe utilized. Specifically, in an X-ray diffraction spectrum of powdersof a catalyst for a fuel cell, peaks derived from Pt and Ir preferablyare not present substantially, and only a peak derived from a Pt-Iralloy preferably is present substantially between peaks derived from Ptand Ir. For example, as in an X-ray diffraction spectrum (FIG. 3)obtained in Example described later, single peak is present at 2θ=81 to82°, and half bandwidth thereof preferably is within 1°, and morepreferably is within 0.8°.

According to a method of the present invention, particle diameter of theplatinum-iridium alloy particles supported on the conductive carrier canbe made small. Specifically, average particle diameter of theplatinum-iridium alloy particles supported on the conductive carrier canbe made preferably not larger than 5 nm. Average particle diameter ofthe alloy particles can be measured by observation image with atransmission electron microscope.

Supporting amount of the platinum-iridium alloy particles preferably is1 to 50% by mass, and more preferably is 1 to 30% by mass based on totalmass of the catalyst for a fuel cell. Supporting amount less than 1% bymass could not provide a catalyst having desired catalytic activity. Onthe other hand, supporting amount over 50% by mass is too high, andcould not provide catalytic effect comparable to the supporting amount,due to overlapping of the alloy particles.

When transition metal is included in the alloy particles, content of thetransition metal is not especially limited, however, preferably is equalto or smaller than 30% by mass, andmore preferably is equal to orsmaller than 10% by mass based on total mass of the catalyst for a fuelcell. Lower limit thereof is also not especially limited, however,preferably equal to or smaller than 1% by mass is contained tosufficiently draw out effect by inclusion of other component.

According to a method for producing the catalyst for a fuel cell of thepresent invention, the fine particle aggregate is prepared in advance,wherein platinum and iridium are admixed in highly dispersed state,using a an inverted micelle method, and then this aggregate is supportedon a carrier. Therefore, the catalyst for a fuel cell, wherein theplatinum-iridium alloy particles having homogeneous compositions aresupported on a conductive carrier in highly dispersed state, can beobtained. Furthermore, because the fine particle aggregate, whereinplatinum and iridium are admixed in highly dispersed state in advance,is used, firing temperature can be lowered also and coarsening of theplatinum-iridium alloy particles by firing can be suppressed, resultingin enhanced catalytic activity.

According to such a method of the present invention, particle diameterof the platinum-iridium alloy particles supported on the conductivecarrier can be made small, by which the catalyst for a fuel havingexcellent catalytic activity and uniform composition of the alloyparticles can be provided. By using the catalyst for a fuel, a fuel cellfulfilling excellent performance of electric power generation stably canbe provided.

The catalyst for a fuel cell produced by a method of the presentinvention preferably is used for an electrode catalyst in an electrodecatalyst layer participating an electric power generation reaction of afuel cell. In addition, an electrode catalyst layer for which thecatalyst for a fuel cell can be used may be arranged at either of theanode side or the cathode side.

Furthermore, because a fuel cell using the catalyst for a fuel cell ofthe present invention described above has the high performance ofelectrical power generation, technological performance of a vehicle canbe improved by using the fuel cell as power source of a vehicle such asan automobile.

EXAMPLE 1

Using 66 g of polyethylene glycol mono-4-nonylphenyl ether as asurfactant, and cyclohexane as an organic solvent, 1.0 L of the solution“A” having a surfactant concentration of 0.15 mol/L, was prepared.

Separately, the aqueous solution “B” containing an iridium compound wasprepared according to the following procedure. First, 10 g of 1.2% bymass hexachloroiridium acid (H₂IrCl₆), as a raw material of an iridiumcompound, and 26 g of a 0.4% by mass aqueous solution of sodiumhydroxide were mixed. In this time, the molar ratio of iridium chlorideto sodium hydroxide was 6.1. Subsequently, the mixed solution was mixedand stirred at 40° C. for 2 hours on a hot stirrer to prepare thesolution “B”. Concentration of iridium chloride in the solution “B” was0.33% by mass.

6.4 g of the prepared solution “B” was prepared was added into thesolution “A” and stirred for 30 minutes to prepare the solution “C”containing an inverted micelle consisting of the iridium compoundclathrated by a surfactant.

Then, 0.5% by mass aqueous hydrochloric acid, as a precipitating agent,was added into the solution “C” and stirred for 30 minutes forinsolubilization treatment of the iridium compound to deposit a fineiridium particle aggregate composed of an iridium hydroxide hydrate,Ir(OH)₃(H₂O)₃.

Added amount of the aqueous hydrochloric acid was such level as requiredto make pH of the solution “B” about 7.

8.1 g of an aqueous solution of dinitrodiamineplatinumnitrate, as anaqueous solution containing a platinum compound, was poured into thesolution “D” containing the fine iridium particle aggregate and stirredfor 30 minutes to impregnate the solution containing the platinumcompound inside the fine iridium particle aggregate. Concentration ofplatinum in the aqueous solution containing the platinum compound was 1%by mass.

0.2 g of sodium borohydride (NaBH₄), as a reducing agent, was graduallyadded into the solution “E” poured into the aqueous solution containingthe platinum compound in several portions under stirring, and furtherstirred for 2 hours to deposit platinum metal inside the fine iridiumparticle aggregate.

By this procedure, the solution “F” containing the fine iridium particleaggregate highly dispersed with platinum metal was obtained.

Separately, 0.43 g of carbon black, as a conductive carrier,(Ketjenblack™ EC600JD produced from Ketjen Black International Co.,Ltd.; BET specific surface area=1270 m²/g) was added into 200 mL ofcyclohexane and subjected to ultrasonic dispersion for 3 minutes toprepare a carbon black dispersed solution “G”.

The carbon black dispersed solution “G” was gradually poured into thesolution “F” in several portions, and then stirred for 1 hour to preparethe solution “H”. Subsequently, 50 ml of methanol was added to thesolution “H” to collapse the inverted micelle, stood still overnight andthe solid content was separated by filtration. The solid content wasdried at 85° C. for 12 hours under reduced pressure, fired at 630° C.for 1 hour in helium gas flow to yield the powdery catalyst for a fuelcell “A” composed of carbon black, as a conductive carrier, and theplatinum-iridium alloy particles supported on carbon black. Supportingamount of the platinum-iridium alloy particles in the catalyst for afuel cell was 20% by mass. Average particle diameter of theplatinum-iridium alloy particles was 4.5 nm.

COMPARATIVE EXAMPLE 1

50 ml of ethanol, as a reducing agent, and 50 g of an aqueous solutionof dinitrodiamine platinum nitrate, having a platinum concentration of0.5% by mass, as raw material of catalyst metal particles, were added to0.43 g of carbon black, as a conductive carrier, (Ketjenblack™ EC600JDproduced from Ketjen Black International Co., Ltd.; BET specific surfacearea=1270 m²/g) and mixed under stirring.

The mixed solution was maintained at 85° C. for 6 hours while mixingunder stirring to proceed a reduction reaction until the solution becamecolorless and transparent. Then, solid content was separated byfiltration, and washed with pure water several times. The solid contentwas further dried at 80° C. for 8 hours to yield the carbon powder “B”supporting platinum.

The carbon powder “B” supporting platinum was added into a solution ofhexachloroiridium acid (H₂IrCl₆) with an iridium concentration of 1% bymass, and after 1 hour of stirring, the added solution was dried at 90°C. under reduced pressure using a rotary evaporator to yield the powder“C”. The powder “C” was further dried at 85° C. for 12 hours underreduced pressure, fired at 630° C. for 1 hour in helium gas flow toyield the powdery catalyst for a fuel cell “C” composed of carbon black,as a conductive carrier, and the platinum-iridium alloy particlessupported on carbon black. Supporting amount of the platinum-iridiumalloy particles in the catalyst for a fuel cell was 20% by mass.

(Evaluation by a Powder X-ray Diffraction Method)

Powder X-ray diffraction spectra were measured on the catalysts for afuel cell supporting the platinum-iridium alloy particles, preparedaccording to Example 1 and Comparative Example 1. FIG. 3 shows X-raydiffraction spectra of the catalyst for a fuel cell prepared in Example1 and the catalyst for a fuel cell prepared in Comparative Example 1.

Comparison of peaks derived from Pt, Ir and alloys thereof, observed atthe vicinity of 2θ=80° to 84°, shows that X-ray diffraction spectrum ofthe catalyst of Comparative Example 1 has broad peak shape, suggestingthat the platinum-iridium alloy particles are supported and dispersed inheterogeneous composition state. On the other hand, peak shape of X-raydiffraction spectrum of the catalyst of Example 1 is sharp, suggestingthat the platinum-iridium alloy particles are supported and dispersed inhomogeneous composition state.

As shown above, the catalyst in Comparative Example has heterogeneousalloy particle composition compared with the catalyst of the presentinvention. On the other hand, the catalyst of the present invention hashomogeneous alloy particle composition and capable of effectivelyfulfilling characteristics as an alloy. To make composition of alloyparticles of the catalyst of Comparative Example 1 close to homogeneousstate, treatment such as firing at high temperature is required,however, firing at high temperature could increase particle size andlower catalytic activity. In addition, firing at high temperature couldincur worse production cost or energy efficiency.

In summary, according to the present invention, such a platinum-iridiumcatalyst can be provided that has small particle diameter, excellentcatalytic activity, along with homogeneous alloy particle composition,and is capable of sufficiently expressing characteristics as an alloy.In addition, firing temperature can also be made relatively low.

Examples described above are those for more specifically explaining thepresent invention, and the present invention should not be limitedthereto.

The present application is based on Japanese patent application No.2004-134207 filed in Japan on Apr. 28, 2004, and disclosed contentsthereof are herein incorporated, by reference in its entirety.

1. A method for producing a catalyst for a fuel cell comprising: a stepfor forming an inverted micelle consisting of an aqueous solutioncontaining the iridium compound clathrated by a surfactant, by mixing anorganic solvent containing said surfactant, and the aqueous solutioncontaining said iridium compound; a step for forming a fine iridiumparticle aggregate by insolubilization treatment of said iridiumcompound; a step for impregnating said fine-iridium particle aggregatewith an aqueous solution containing a platinum compound; a step forobtaining a solution containing the inverted micelle clathrating thefine iridium particle aggregate containing platinum by reducing saidplatinum compound and depositing platinum metal in said fine iridiumparticle aggregate; a step for supporting said fine iridium particleaggregate containing platinum on a conductive carrier by dispersing saidconductive carrier in said solution; and a step for firing theconductive carrier whereon said fine iridium particle aggregatecontaining platinum is supported.
 2. The method for producing a catalystfor a fuel cell of claim 1, wherein said iridium compound is an iridiumcomplex.
 3. The method for producing a catalyst for a fuel cell of claim1, wherein concentration of said iridium compound in the aqueoussolution containing said iridium compound is 0.1% by mass to 30% bymass.
 4. The method for producing a catalyst for a fuel cell of claim 1,wherein the aqueous solution containing said iridium compound isprepared by mixing iridium chloride and a basic compound containing ahydroxyl group of 5 to 10 times moles of iridium atoms contained in saidiridium chloride, and then by causing a reaction at 30 to 50° C. for 1to 5 hours.
 5. The method for producing a catalyst for a fuel cell ofclaim 1, wherein the step for forming said fine iridium particleaggregate is a step for forming the fine iridium particle aggregate byadding a precipitating agent to the aqueous solution containing saidiridium compound.
 6. The method for producing a catalyst for a fuel cellof claim 5, wherein said precipitating agent is one or more kinds ofacids selected from a group comprising hydrochloric acid, nitric acid,sulfuric acid, and acetic acid.
 7. The method for producing a catalystfor a fuel cell of claim 1, wherein said platinum compound isPt(NO₂)₂(NH₃)₂ or H₂PtCl₆.
 8. The method for producing a catalyst for afuel cell of claim 1, wherein the step for depositing platinum metal insaid fine iridium particle aggregate is a step for depositing platinummetal in said fine iridium particle aggregate by reducing said platinumcompound using a reducing agent.
 9. The method for producing a catalystfor a fuel cell of claim 8, wherein said reducing agent is one or morekinds selected from a group comprising N₂H₄, NaBH₄ and H₂ gas.
 10. Themethod for producing a catalyst for a fuel cell of claim 1, furthercomprising, before the step for supporting said fine iridium particleaggregate containing platinum on said conductive carrier: a step forimpregnating said fine iridium particle aggregate containing platinumwith an aqueous solution containing transition metal; and a step fordepositing said transition metal in said fine iridium particle aggregatecontaining platinum.
 11. The method for producing a catalyst for a fuelcell of claim 10, wherein said transition metal is one or more kindsselected from a group comprising chromium, manganese, iron, cobalt,nickel, rhodium and palladium.